Project Summary/Abstract Allogeneic hematopoietic cell transplant (HCT) recipients represent a defined population in which approximately 50% of patients will develop new-onset post-transplant diabetes mellitus (PTDM) and in whom the development of diabetes increases the risk of death 3-fold. The propagation of pre-diabetes to frank hyperglycemia occurs when pancreatic ?-cells can no longer meet the insulin demand needed for glucose homeostasis. Loss of IL-33/serum STimulation-2 (ST2) signaling and depletion of ST2+ regulatory T cells (Tregs) in visceral adipose tissue exacerbates ?-cell exhaustion by increasing both Th1-mediated inflammation and insulin resistance. In a cohort of HCT recipients, we demonstrated that PTDM development was characterized by: 1) elevated fasting C-peptide levels prior to transplant; 2) impaired islet response to hyperglycemia and GLP-1 after HCT with decreased ?-cell insulin secretion and blunted ?-cell suppression, and 3) increased post-transplant serum levels of soluble ST2 (sST2), a decoy receptor which sequesters IL-33. We hypothesize that in PTDM, initial ?-cell compensation progresses to exhaustion during the course of HCT, which coincides with increased tissue demand for insulin due to changes in IL-33 signaling, inflammation, and/or hyperglucagonemia. The following aims will test islet cell and ST2 regulation during PTDM. Aim 1. To determine if changes in islet cell physiology are detectable before or after matched related donor (MRD) HCT in patients developing new-onset PTDM. Utilizing a hyperglycemic clamp, we will directly measure insulin secretory capacity before and 90 days after MRD HCT to determine the timing and role of ?-cell dysfunction in the development of new-onset PTDM (Subaim 1A). To assess ?-cell dysregulation, glucose-induced glucagon suppression will be measured during a hyperglycemic clamp and during 2 oral glucose tolerance tests either with or without GLP-1 infusion (Subaim 1B). In Aim 2 we will define the role of the IL-33/ST2 axis in immune/islet cell dysregulation during PTDM by measuring adipose and plasma levels of IL-33 and sST2 and quantifying ST2 expression on circulating Tregs and Th1 cells. IL-33 effects will be assessed in vitro to determine whether T cell proliferation or inflammatory cytokine production differs among patients with or without PTDM or whether IL-33 directly decreases human islet insulin secretion and viability. PTDM is highly prevalent in HCT survivors, however the cause, pathophysiology, and optimal management are unclear. By studying the physiology and immunology of PTDM, this proposal will uncover new connections between metabolic complications and immune regulation while simultaneously identifying novel targets for intervention. Longer term, data from these mechanistic studies will be translated into therapeutic clinical trials to test pharmacologic interventions for the prevention and treatment of PTDM.